Valve-gated injection molding devices are well-known in the art. These devices use a reciprocating valve pin seated within a bushing to open and close the front opening of a nozzle, thereby establishing or cutting off the flow of material out the nozzle. One common problem that is faced by these valve-gated injection molding devices is the seepage of excess material backward into the bushing, rather than out of the nozzle. The seepage of excess material into the bushing, or worse yet, through the bushing and into the actuating mechanism that drives the valve pin, can cause the valve pin to lock-up or jam, and render the injection molding device inoperable.
In order to avoid this problem, prior art injection molding devices have typically used a tight fit between the valve pin and its bushing to block the leakage of excess material. The tight fit between the valve pin and its bushing, however, usually results in the break down of the melt material caused by the shearing action from the reciprocating motion of the valve pin on the material confined in the small space between the valve pin and its bushing. Consequently, the breakdown causes the formation of hot pressurized gases and acids that attack the surface of the valve pin and its bushing. The resulting corrosion, wear, and build-up of solidified material usually results in seizure of the valve pin, thereby shortening the operative life of the injection molding device.
One solution to the problems caused by this seepage of excess material into the bushing is disclosed by U.S. Pat. Nos. 4,026,518, 4,433,969, and 5,387,099 to Gellert. Each of the injection molding devices disclosed in these patents has a circumferential opening in its bushings that is vented to the atmosphere through one or more ducts in the bushings and the manifold retaining the bushings. The vented circumferential opening relieves the pressure on the seeping material, allows harmful gases to escape, and permits the excess material trapped between the valve pin and its bushing to remain partially fluid. While this vented circumferential opening has been found to substantially extend the operative life of the injection molding device, providing such a vented circumferential opening, especially its ducts, involves a complex and expensive drilling operation into both the bushing and its corresponding manifold. Moreover, the separate individual ducts do not provide an efficient central channel or passage system for collecting and disposing of the seeping material.
Accordingly, it would be desirable to provide a seepage system that relieves excess material and harmful gases, but does not involve complex and expensive drilling operations. It would also be desirable to provide a seepage system with an efficient central seepage passage for collecting and disposing of seeping material.